This idea comes from a reddit user edtheloon. When I posted my post about screenfetch and linux_logo on reddit, he commented that I fotgot the most important thing that I can add them into .bashrc. I have to say that is a awesome idea. .bashrc file is very uesful that you can add your personal environment variables in it.
In order to make a command can be execute after your login, you can just put the command in the file:
vim ~/.bashrc # put them anywhere except inside if screenfetch linux_logo #in some systems it's linuxlogo
Save and exit. You can try to log out and login again. I tested this on my computer, it works.